Control of cement clinker production by analysis of sulfur in the end product

ABSTRACT

Cement clinker is produced using high sulfur fuels by combusting high sulfur fuel within a kiln. The feed material is introduced at an inlet of the kiln and is sintered by the combusting fuel to form sintered material. The sintered material is cooled to form cement clinker. The production of the sintered material is controlled by measuring the sulfur content in the cement clinker and using that measurement to control the concentration of oxygen at the inlet of the kiln.

BACKGROUND

The invention is related to producing cement clinker.

In known processes for producing cement clinker, raw material fed into arotary kiln is preheated and partially decarbonated in a multistagecyclone suspension preheater system and a precalciner by using the heatof combustion gases exhausted from the rotary kiln and precalciner. Asthe combustion gases and raw material mix, lime (CaO) in the rawmaterial and sulfur dioxide (SO₂) in the combustion gases react to formcalcium sulfite (CaSO₃). The calcium sulfite is formed in the preheaterand in the main electrostatic precipitator of the stack. The calciumsulfite, in turn, reacts with oxygen in the preheater system to formcalcium sulfate (CaSO₄), if there is sufficient oxygen. If there is notenough oxygen in the atmosphere at the kiln's inlet, the calcium sulfatemay decompose into lime and sulfur dioxide and leave depositions at thekiln's inlet. If there is an insufficient excess of oxygen in the rotarykiln, the calcium sulfate may decompose at temperatures of 1200°Celsius. Similarly, if there is not enough oxygen in the preheaters, thecalcium sulfite may decompose into lime and sulfur dioxide. Thisdecomposition also leads to an increase in sulfur dioxide concentrationin the gas in the kiln, which leads to depositions of calcium salts onthe shells and walls of the preheater's cyclones and ducts. The level ofdeposit formation may be increased when the combustion fuel is a solidfuel high in sulfur (i.e., above 2%), such as petcoke, oil shale, andagricultural or industrial wastes, or a fuel oil high in sulfur contentbecause of the resulting increased sulfur dioxide concentration in thekiln gas. The increased sulfur circulating in the gases causes anincrease in the quantity of calcium sulfite. This may result in depositsto a level sufficient to close the kiln inlet, preheater, preheatercyclones, and ducts connecting the cyclones, thereby stoppingproduction. The problem can be alleviated by extracting a fraction ofthe gas between the rotary kiln and preheater and sending it to a bypasstower. In the bypass tower, the gas is quenched with cooler atmosphericair and a dust rich in sulfur dioxide precipitates out. The desulfurizedgas is then directed into the preheater, the result being an overallreduction in the concentration of sulfur dioxide in the gas in thepreheater. This solution poses two significant problems: a loss inthermal energy and an environmental issue in disposing of theprecipitated dust.

Alternatively, the oxygen can be controlled to ensure an excess oxygenconcentration in the kiln and eliminate the need for a bypass tower.However, this potential solution is prone to problems associated withoxygen sensor reliability in a kiln environment, which is furtherreduced at the kiln inlet where oxygen concentration is even moreimportant. At the inlet, the gas intake for oxygen analyzers can befilled by the dust circulating in the kiln environment. Because currentoxygen sensors in the kiln environment may be unreliable, it is notpractical to provide continuous control of cement clinker productionusing an oxygen sensor. To provide excess oxygen by merely increasingthe flow of air through the kiln, precalciner, and preheaters may createother problems associated with reduced thermal efficiency and pressureloss.

SUMMARY

The invention provides a process having an air intake rate that isregulated based on the quantity of calcium sulfate measured in thecement clinker end product as sulfur or sulfur trioxide (SO₃). The airintake rate directly affects the amount of oxygen in the kiln that isavailable for the reaction converting CaSO₃ to CaSO₄, and also affectsthe rate at which they decompose. An increase in the concentration ofoxygen to 4.5 to 5.5% increases the temperature at which calcium sulfatedecomposes to a temperature greater than the sintering temperature suchthat CaSO₄ becomes a component of the finished product rather thandecomposing into gases and leaving deposits in the kiln, preheater, andpreheater cyclones. Thus, analysis of the sulfur in the cement clinkerend product can be used to control the oxygen concentration in the kilnand thereby indirectly control the proportion of sulfur exiting thesystem as part of the cement clinker.

The air intake to the kiln is mechanically adjusted by increasing ordecreasing the speed of a main exhauster that creates a negativepressure that pulls air into and through the kiln, preheater, preheatercyclone's, and precalciner. The air carries the combusted fuel gasesfrom the kiln and precalciner into the preheater. In the preheater andpreheater cyclones, the raw material is preheated and separated from thegases. It also is partially precalcined, i.e., the calcium carbonate inthe raw material is partially decomposed into lime and carbonic (CO₂)gas. In the precalciner, the raw material is further decarbonated to alevel of 90 to 95%. In addition, the gas is desulfurized in the mainelectrostatic precipitator of the stack and preheater by transfer of thesulfur in the gas to the raw material through the reaction CaO+SO₂→CaSO₃. Thus, 90 to 95% of the carbonic gas in the raw material isreleased before the raw material reaches the kiln inlet.

Control of the air intake may be accomplished when using a rotary kilnfor producing the cement clinker. The raw material enters the system asa whole at the upper end of the preheater and enters the rotary kilnthrough an inlet at the kiln's upstream end, which is connected to thepreheater outlet. The inlet also contains a vertical connection to theprecalciner through which passes the combustion gases produced byburning fuel at the rotary kiln's burner. The burner, located at thedownstream end of the rotary kiln, produces the heat needed forsintering the raw materials in the kiln. The kiln is inclined tofacilitate the flow of material. After the cement clinker passes throughthe kiln's sintering zone, it exits the rotary kiln to the coolerthrough an outlet adjacent to the burner. The outlet for the cementclinker also serves as an inlet to the rotary kiln for a portion of theair that is blown into the cooler to cool the cement clinker. The air isheated as it cools the cement clinker. The air is blown into the coolerby multiple fans and creates an increase in pressure in the cooler.

The cooling air not flowing into the kiln exits the cooler through twooutlets. One outlet directs the air into an electrostatic precipitatorto recover fines of the clinker, after which the air is released intothe atmosphere. The other outlet directs the air into a dust chamberthat returns clinker dust to the cooler and directs the air into theprecalciner. A valve on the line between the dust chamber andprecalciner regulates the flow of air into the precalciner and affectsthe proportion of air flowing through these two lines and the kiln. Asless air is directed to the precalciner by closing the valve, more airflows through the kiln and electrostatic precipitator of the cooler.

The precalciner decarbonates the raw material using the combustion gasesfrom the rotary kiln and by combusting fuel at a burner in theprecalciner. The oxygen for the combustion is supplied as a component ofthe heated air entering the precalciner from the rotary kiln and throughan air inlet connected to the tertiary air duct and located at the baseof the precalciner. The raw material feeds into the precalciner from thedust outlet of a cyclone suspension preheater.

The invention permits a more economical use of solid, liquid or gaseoushigh sulfur fuels in the production of cement clinker in rotary kilns.The invention also permits operating conditions to be maintained so thatthe sulfur in the fuel is transferred to the cement clinker in the formof CaSO₄, which drastically reduces the SO₂ concentration in the processand thereby reduces SO₂ emissions. The invention improves the process ofproducing cement clinker by permitting the use of fuels containing up to10% sulfur and reducing the emissions of SO₂ and NO_(x) gases. The 10%sulfur limit is based on using fuels with calorific values ofapproximately 8,000 kilocalories per kilogram of fuel. The NO_(x) gasemissions are reduced by creating a reducing atmosphere that uses the O₂of the NO_(x) in the precalciner. Additionally, if there is enoughcalcium sulfate in the clinker, no additional gypsum needs to be addedto act as a cement setting retarder while grinding the clinker forcement production.

In one general aspect, the invention may produce cement clinker usinghigh sulfur fuels combusted in a kiln into which feed material isintroduced at an inlet of the kiln. The feed material is sintered toform sintered material, which is cooled to form cement clinker. Theprocess is controlled by measuring the sulfur concentration in thecooled cement clinker at the cement clinker cooler outlet to control theoxygen concentration at the inlet to the kiln.

Embodiments may include one or more of the following features. Forexample, the feed material may be precalcined using high sulfur fuelscombusted in the precalciner and preheated using the combustion gasesfrom the kiln and precalciner. The oxygen concentration in theprecalciner may also be controlled to help use the excess oxygen fromthe kiln and create a reducing atmosphere in the precalciner to reduceNO_(x) emissions. The oxygen concentrations in the precalciner and kilnmay be varied by adjusting the speed of an exhauster that draws airthrough the kiln, preheater, and precalciner. A valve positioned in aline between the precalciner and cooler also may be adjusted by acontroller to vary the amount of air flowing into the kiln andprecalciner.

The oxygen concentration may be controlled to maintain an elevateddecomposition temperature of calcium sulfate in the kiln to preventcalcium sulfate decomposition. The fuel used in the burners may containup to 10% sulfur and the sulfur in the fuel reacts with the CaO of thefeed material to form calcium sulfate, which becomes a component of thecement clinker up to weight concentrations of 3%. Because of the calciumsulfate in the cement clinker, no gypsum needs to be added whilegrinding the cement clinker to produce cement. By controlling the oxygenin the kiln, the circulation of SO₂ in the kiln can be reduced to lessthan 80 kg per hour to eliminate deposits in the kiln, cyclones, andducts connecting the cyclones.

Embodiments also may include a rotary kiln with a burner to sinter theraw material, a cooler to cool the cement clinker, a sulfur analyzer tomeasure the sulfur content in the cement clinker and a controller tocontrol the oxygen concentration in the kiln based on the measuredsulfur content of the cooled cement clinker. The controller may use themeasured sulfur content to control the speed of an exhauster to controlthe oxygen concentration in the kiln. The kiln may be connected to aprecalciner that burns high sulfur fuels to precalcine (i.e.,decarbonate) the raw material. A cyclone suspension preheater may beconnected to the kiln and precalciner to preheat and partiallydecarbonate the raw material before it enters the kiln. A tertiary airline and valve between the cooler and precalciner may be used toregulate the flow or air to the precalciner. Oxygen sensors may beplaced at the kiln inlet and gas outlet from the cyclone suspensionpreheater to monitor oxygen. The oxygen concentration at the gas outletof the cyclone suspension preheater may be used by the controller tocontrol the tertiary air line valve. An electrostatic precipitator maybe used to filter the air passing from the cooler to the atmosphere.

Other features and advantages will be apparent from the followingdetailed description, including the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for producing cement clinker.

FIG. 2 is a diagrammatic view of cyclone suspension preheaters, aprecalciner and a rotary kiln inlet section of the system of FIG. 1.

FIG. 3 is a diagrammatic view of a rotary kiln, precalciner, cementclinker cooler, tertiary air duct line, dust chamber, and electrostaticprecipitator of the cooler of the system of FIG. 1.

FIG. 4 is a block diagram of a control system for the system of FIG. 1.

FIG. 5 is a flow diagram showing the flow of material through thecyclone suspension preheaters, precalciner, and rotary kiln inlet of thesystem of FIG. 1.

FIG. 6 is a block diagram of the main exhauster and associated equipmentof the system of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, a system for producing cement clinker includes arotary kiln 25 and a cement clinker cooler 30 positioned at an outlet 35of the kiln. A tertiary air duct line 40 connects the cement clinkercooler to a precalciner 45. A gas exhaust line 20 connects the rotarykiln and precalciner. A multiple cyclone suspension preheater system 50is connected to a rotary kiln inlet 55, to the precalciner 45, and to anoutlet line 60 that leads to an exhauster 65.

The raw material is supplied to the system at an inlet pipe 67. The rawmaterial mixes with combustion gases produced at a rotary kiln burner 69and a precalciner burner 70. The combustion gases are pulled through thekiln 25, suspension preheater system 50 and precalciner 45 by theexhauster 65. The raw material is heated by and separated from thecombustion gases in the cyclones of the suspension preheater. The heatedraw material flows into the precalciner where the extent ofdecarbonation is increased. The raw material then flows into the bottomcyclone of the suspension preheater where it is separated from thecombustion gases and flows into the inlet of the rotary kiln 25.

The raw material is sintered as it flows through the rotary kiln to formcement clinker. The cement clinker then flows into the cement clinkercooler through kiln outlet 35. The cement clinker is cooled by coolingair blown across its grate. The cooling air then flows into the rotarykiln 25, the tertiary air duct line 40, and an electrostaticprecipitator 73. The electrostatic precipitator filters the excess ofair of the cooler before releasing it into the atmosphere.

The flow of oxygen into the kiln and precalciner is controlled by theexhauster that pulls air through the system. The flow of oxygen into theprecalciner also is controlled by the position of a tertiary air ductvalve 75. The concentrations of sulfur are measured and those values areused by a control box 80 to change the speed of the exhauster.

Two important aspects of the invention are the use of high sulfur fuelat the burners and purging sulfur from the process by discharging it asa component of the cement clinker end product in the form of calciumsulfate. To permit use of high sulfur fuel and to purge sulfur as acomponent of the end product, the speed control of the exhauster 65 isregulated to control the amount of oxygen in the kiln 25. (Excess oxygenprevents the decomposition of calcium sulfate at the sinteringtemperature of the clinker.) The speed of the exhauster is controlledbased on the quantity of sulfur or sulfur trioxide in the cooled cementclinker, as measured by a pair of sulfur analyzers 77 and 79 thatanalyze the clinker at the outlet to cooler 30. The analysis of thesulfur in the cooled cement clinker shows the extent to which the sulfurin the fuel and raw material is being discharged into the clinker. Ifthe sulfur content must be increased, the control box increases thespeed of the exhauster.

As shown in FIG. 2, the preheater 50 may be implemented using asuspension preheater containing four cyclones. As described below, threeof the cyclones heat the raw material before it passes through theprecalciner, and the fourth heats and separates the heated material fromgases before the heated material is fed into the rotary kiln's inlet.

The raw material is fed in at inlet pipe 67 and the majority of thematerial passes through a gas outlet 145 into a cyclone 105. A portionof the material fed in at inlet pipe 67 is carried in the gas flowingout of cyclone 105 into a twin cyclone 110 of suspension preheater 50.Twin cyclone 110 is constructed to separate the fine raw material thatentered it from inlet 67 in the ascending gases. In twin cyclone 110,the cyclone effect separates the gas from most of the raw material intoa gas stream exiting the preheater at a gas outlet 115 and a rawmaterial stream exiting from a dust outlet 120. Gas outlet 115 ofpreheater 50 is connected to intake piping 60 for exhauster 65, whichpulls air through the entire system.

The raw material passes downward through the cyclones of the preheaterwhile the gases pass upward through the cyclones. Dust outlet 120 oftwin cyclone 110 feeds raw material into a line connected to a gasoutlet 135 of a cyclone 125 and an inlet 140 of cyclone 105. The gasstream from cyclone 125 combines with most of the raw material andfurther heats that raw material before the combined streams entercyclone 105. The remainder of the raw material flows into cyclone 125.In cyclone 105, the cyclone effect separates the gas and raw materialinto a gas stream exiting at the gas outlet 145 and a raw materialstream exiting at a dust outlet 150.

Dust outlet 150 feeds the raw material into a line connected to a gasoutlet 160 of a cyclone 165 and an inlet 155 of cyclone 125. Part of theraw material passes into cyclone 165 and part into outlet 170 of cyclone125. The material flowing into cyclone 165 passes through a dust outlet185 into a rotary kiln inlet 55, which is connected by a line 190 toprecalciner 45. A large portion of the hot gases in the rotary kiln 25,and some of the raw material, is sucked through line 190 intoprecalciner 45. The remainder of the raw material passes into rotarykiln 25. In cyclone 125, the cyclone effect separates the gas from themajority of the raw material into a gas stream exiting at a gas outlet135 of cyclone 125 and a dust stream exiting at a dust outlet 170 ofcyclone 125. The dust stream feeds into precalciner 45.

The raw material fed into precalciner 45 through lines 190 and 170 isfurther decarbonated by the heat produced at a secondary burner 70 andcarried in the combustion gases to an inlet 180 of cyclone 165. Incyclone 165, the cyclone effect separates the gas from most of the rawmaterial into a gas stream and a raw material stream. The gas streamflows out of gas outlet 160 of cyclone 165 into inlet 155 of cyclone125. As described above, the raw material stream passes through a dustoutlet 185 of cyclone 165 and feeds into the raw material inlet 55 ofrotary kiln 25.

Referring to FIG. 3, the raw material, which is highly decarbonated, isfed into rotary kiln 25 at rotary kiln inlet 55. The material continuesto flow in the direction of the outlet 35 of kiln 25 and, in theclinkerization zone, is sintered by gas combusted at kiln burner 69. Thesintered material (i.e., cement clinker) flows from rotary kiln 25 intoclinker cooler 30 through kiln outlet 35. Cooled cement clinker flowsout of clinker cooler 30 at a clinker outlet 215. A cooling fans system220 blows cooling air across the cement clinker. The cooling air exitscooler 30 through an excess air exit 225, tertiary air duct line 40, andkiln outlet 35.

The air flowing into tertiary air duct line 40 passes through a dustchamber 235 before flowing into precalciner 45. The dust recovered inthe dust chamber 235 is returned to cooler 30 through a line 280connecting dust chamber 235 to cooler 30. Line 280 contains a counterweight flap 285 to control the flow of dust into the cooler 30. The airflows into precalciner 45 through a pair of tertiary air duct outlets250 and 255. Tertiary air duct valve 75, which is positioned betweendust chamber 235 and precalciner 45, controls the rate of flow of airinto precalciner 45. Adjusting valve 75 also affects the rate of flow ofair through excess air exit 225 into electrostatic precipitator 73 andthrough kiln outlet 35. To provide a slight increase in the oxygenconcentration in the kiln without increasing the speed of the exhauster,valve 75 can be adjusted to send less air to the precalciner based onthe concentration of oxygen at the preheater outlet 60. The air exitinginto kiln 25 through kiln outlet 35 flows through the kiln, exits thekiln through a kiln gas outlet 245 and flows into precalciner 45 throughline 190. The air flowing through the precalciner is pulled by thenegative pressure created by exhauster 65 connected to inlet piping 60.

Referring to FIG. 4, control box 80 controls the speed of exhauster 65through motor controller 310. Control box 80 also controls the positionof the tertiary air duct valve 75. Control box 80 controls the speed ofexhauster 65 to feed enough oxygen to the sintering zone based on thesulfur concentration in the cooled cement clinker at a cooler outlet215. As described above, sufficient oxygen in the sintering zone willprevent calcium sulfate decomposition so that the calcium sulfatebecomes a part of the cooled cement clinker.

The oxygen concentration is measured at duct 60 by an oxygen analyzer335. The value measured by oxygen analyzer 335 is sent to control box80, which regulates the position of tertiary air duct valve 75 to keepthe oxygen level less than 1.5 to 2% in the line to exhauster 65. Thisregulation permits the use of the excess oxygen leaving the sinteringzone of the rotary kiln 25 and part of the oxygen of the NO_(x)pollutants that could have been produced in rotary kiln 25. Control box80 also receives an oxygen concentration value from an oxygen analyzer320, which measures the oxygen concentration at the kiln inlet. Thisvalue is used for recording purposes only because oxygen sensors inenvironments such as a kiln inlet are unreliable.

A carbon monoxide analyzer 315 measures the concentration of carbonmonoxide at duct 60 and sends the value to control box 80. Theconcentration of carbon monoxide at duct 60 is monitored as a means ofpreventing an explosion at a downstream main electrostatic precipitator,which can occur if the carbon monoxide concentration rises too high. Ifthe carbon monoxide concentration measured in duct 60 rises above 0.6%,control box 80 shuts off the flow of fuel to burners 69 and 70. Afterthe condition is corrected, a push button on control box 80 permits fuelto flow to burners 69 and 70. If the concentration of oxygen or carbonmonoxide is measured to be zero, control box 80 gives an alarmindicating that the gas intakes for analyzers 335 and 315 need to becleaned to remove a build up of material.

FIG. 5 further illustrates the flow of material and gases through thepreheater's cyclones and precalciner. The majority of the raw materialintroduced at inlet 67 passes into cyclone 105. Some of the finer rawmaterial is carried to cyclone 110 in the upward draft of the gas fromcyclone 105. As the material passes through the gas, it is heated by thegas. Also, the sulfur dioxide in the gas reacts with the lime in the rawmaterial to form CaSO₃, thereby stripping sulfur dioxide out of the gas.This reaction is not limited to cyclone 105 but occurs in the othercyclones, precalciner 45, and their inlet and outlet lines.

In cyclone 110, the cyclone effect separates the gas from the fine rawmaterial by sending the gas out of the top of the cyclone and the rawmaterial out of the bottom of the cyclone. The gas exiting the cycloneis pulled through the line by the exhauster and vented into a separatesystem, described below, that includes an electrostatic precipitator toremove any remaining dust. The exhauster speed determines the rate atwhich air is pulled into the system and through suspension preheatercyclones 105, 110, 125 and 165 and precalciner 45. The exhauster speedis automatically controlled by control box 80, which adjusts theexhauster's motor speed based on and the measured concentrations ofsulfur in the cement clinker end product.

Referring to FIG. 6, exhauster 65 directs the gas to a mainelectrostatic precipitator 200 from which the gases are removed by anexhauster 205 and sent to a stack 210. The gas from exhauster 65 issplit into two streams: one stream flowing into a main gas cooling tower215 and a second stream flowing into a drying mill 220. Both streams areregulated by a pair of controlling flaps 225 and 230. If the mill isstopped, as in the case in which silo 225 is full, all gases are directto pass through cooling tower 215 before entering electrostaticprecipitator 200. If the drying of the raw material requires passing allgases through drying mill 220, flap 225 can be completely closed andflap 230 can be completely opened. The raw material stream flowing intothe drying mill 220 has a mill feed inlet 235 and a recycle line 240 forreturning coarse material from a separator 245. Drying mill 220 feedsthe gas and dried material into the separator 245, in which the coarseparticles are separated and returned to the drying mill 220. Theremaining material and gas passes through a series of cyclones 250 inwhich the material is separated from the gas. The finished raw groundmaterial is passed to a storage silo 255. The gas is pulled from thecyclones by an exhauster 260 which directs the gas into a line connectedto electrostatic precipitator 200 and which also receives the gas fromthe main gas cooling tower 215. The dust from the electrostaticprecipitator is sent to the storage silo 255 or sent to the kiln feedinlet 67.

Referring to FIG. 5, the cyclone suspension preheater heats the rawmaterial and separates it from the gas. Cyclone 125 and, to a lesserextent, line 190 feed heated raw material into precalciner 45 where thematerial is almost completely calcined and decarbonated, and carriedinto cyclone 165 by combustion gases produced at burner 70 ofprecalciner 45. In cyclone 165, the gas and raw material are separatedinto two streams. The gas stream exits through gas outlet 160 into theinlet of cyclone 125. The heated raw material exiting cyclone 165 flowsinto inlet 55 of rotary kiln 25. In addition to heating the rawmaterial, the cyclones and connecting ducts serve as reaction vesselsfor reacting lime (CaO) in the raw material with sulfur dioxide (SO₂) inthe combustion gases and raw material to form CaSO₃. This reactionremoves SO₂ from the exhaust gases before they are vented to theatmosphere by exhauster 65.

In rotary kiln 25, CaSo₃ in the raw material is oxidized to CaSO₄ in thepresence of oxygen. The reaction's equilibrium is controlled by theamount of oxygen in rotary kiln 25. An increase in oxygen concentrationshifts the reaction equilibrium to favor CaSO₄ production. The increasein oxygen also increases the temperature at which CaSO₄ will decompose,from 1200° C. to above 1500° C.

Controlling this reaction has four directly related benefits. First,because CaSO₄ does not decompose in the kiln, it remains a component ofthe cement clinker product and reduces or eliminates the need to addgypsum to the final product. Second, SO₂, a decomposition product, doesnot become a component of the exhaust gases vented by exhauster 65,which reduces sulfur emissions of the system. Third, because the sulfurcontained in the fuel becomes a component of the cement clinker, theburners in the kiln and precalciner can be operated using fuels withsulfur contents as high as 10% before there is too much sulfur in thesystem and CaSO₃ and CaSO₄ depositions build up in the vessels. Finally,precalciner burner 70 creates a reducing atmosphere that decomposesNO_(x) to nitrogen, thereby reducing emission of this pollutant.

Historically, the oxygen concentration in kiln inlet 55 and at asintering zone 270 has been maintained at 1.0 to 1.5%, with a 2%maximum. By increasing the oxygen concentration in those regions to 4.5to 5.5%, the temperature at which calcium sulfate will decompose israised above 1500° C., which is above the operating temperature in thekiln. Although increasing the air flow through the kiln increases theoxygen concentration in the kiln, an indiscriminate increase in air flowmay cause great thermal losses because the air passing through the kilnremoves heat generated by the combusting fuel; the greater the air flow,the greater the thermal and pressure losses. To increase the air flowjust enough to provide the minimum oxygen concentration necessary toprevent the calcium sulfate from decomposing, i.e., 4.5 to 5.5%, theinvention uses a control system based on end product sulfurconcentration. To increase the oxygen concentration to the extentnecessary and take advantage of the elevated decomposition temperature,the control box increases the amount of air flowing through kiln 25, anddecreases the air flow to precalciner 45. Decreasing the air flow to theprecalciner compensates for the pressure and thermal losses in the kiln.The control box increases the air flow to raise the oxygen concentrationto 4.5 to 5.5% by increasing the speed of exhauster motor 65. Byproviding excess oxygen at sintering zone 270 to reduce CaSO₄decomposition, the control box reduces the amount of sulfur dioxide inthe preheater and precalciner. Reducing the amount of sulfur dioxide, inturn, reduces emissions and prevents depositions in the equipment.

The amount of excess oxygen necessary to shift the reaction to CaSO₄ iscontrolled based on the amount of sulfur entering the system in the fueland raw material and the amount of CaSO₄ that can be a component of thefinished product (e.g., a maximum concentration of 3% CaSO₄). Using theknown rate of fuel consumption per ton of cement clinker and theconcentration of sulfur in the fuel, the quantity of sulfur that can beconverted into CaSO₄ can be calculated by the control box. If there issulfur in the raw material, that value must be added to the amount ofsulfur that can be converted into CaSO₄.

Using 3% as the maximum amount of CaSO₄ that is acceptable in one ton ofcement clinker end product, the control box measures the amount ofsulfur in the cement clinker, in the form of CaSO₄, to determine theextent to which the CaSO₄ is decomposing and the So₂ is passing throughthe system. If the CaSO₄ is decomposing in the system, as indicated by acement clinker sulfur content less than the needed quantity to purge thesystem of sulfur, the control box increases the exhauster motor's speedto increase the amount of oxygen in the kiln. The amount of sulfur inthe cement clinker end product is measured by separate sulfur analyzers77 and 79. As a backup, the oxygen concentration at kiln inlet 55 ismeasured and the control box records the fluctuations in oxygenconcentration at the kiln inlet.

As an example of the operation of the system, if the fuel used in theburners is petroleum coke containing 10% sulfur with a caloric value of8,000 kilocalories per kilogram, the consumption of fuel would beapproximately 100 kg of petroleum coke per metric ton of clinkerproduced. Because 100 kilograms of 10% sulfur petroleum coke contains 10kilograms of sulfur and the ratio by weight of sulfur to sulfur trioxide(SO₃) is 32 to 80, 10 kilograms of sulfur can react to form 25 kilogramsof sulfur trioxide.

If the entire 25 kilograms of sulfur trioxide is contained in one metricton of cement clinker, the clinker contains 2.5% sulfur trioxide--aconcentration compatible with international norms. If the raw materialcontains sulfur, the fuel must contain proportionally less sulfur toprevent the concentration of sulfur trioxide in the cement clinker fromexceeding 2.5-3%.

A drop in the concentration of sulfur trioxide in the cement clinker atthe cooler outlet indicates that there is insufficient oxygen in thekiln and precalciner and that calcium sulfate is decomposing. To correctthis, the controller uses the value of the concentration of sulfur inthe cement clinker produced to increase the speed of the exhauster andfurther close the tertiary air duct line valve to provide additional airto the kiln, depending on the oxygen concentration at outlet line 60.The oxygen in the additional air increases the decomposition temperatureof the calcium sulfate, thereby reducing the decomposition of calciumsulfate in the kiln and causing that calcium sulfate to become part ofthe cement clinker. The end result is an increase in sulfur trioxide inthe cement clinker to 2.5%.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. A method of producing cement clinker using highsulfur fuel, comprising:combusting high sulfur fuel within a kiln;introducing feed material at an inlet of the kiln; passing the feedmaterial through the kiln in the presence of the combusting fuel so asto sinter the feed material to form sintered material; cooling thesintered material to form cement clinker; measuring sulfur content ofthe cement clinker; and controlling a concentration of oxygen at theinlet of the kiln based on the measured sulfur content.
 2. The method ofclaim 1, further comprising precalcining the raw material in aprecalciner using high sulfur fuel combusted in the precalciner.
 3. Themethod of claim 2, further comprising preheating the raw material usingcombustion gases from the kiln and precalciner.
 4. The method of claim2, further comprising controlling a concentration of oxygen in theprecalciner.
 5. The method of claim 4, further comprising maintaining areducing atmosphere in the precalciner to reduce NO_(x) emissions. 6.The method of claim 4, further comprising varying the oxygenconcentration in the kiln and precalciner by adjusting a speed of anexhauster that draws air through the kiln and precalciner and varyingthe oxygen concentration in the precalciner by adjusting a valvepositioned between the cooler and the precalciner.
 7. The method ofclaim 1, further comprising controlling the oxygen concentration tomaintain a decomposition temperature of CaSO₄ in the kiln atapproximately 1500° Celsius and above to prevent decomposition of CaSO₄in the kiln.
 8. The method of claim 1, wherein the fuel contains up to10% sulfur.
 9. The method of claim 1, wherein the cement clinkercontains sufficient levels of calcium sulfate so that gypsum need not beadded while grinding the cement clinker to produce cement.
 10. Themethod of claim 1, further comprising maintaining a weight percentage ofSO₃ in the cement clinker at approximately 3% or less.
 11. The method ofclaim 2, further comprising maintaining a circulation of SO₂ in theprocess at less than 80 kg per hour to eliminate calcium sulfate andcalcium sulfite deposits in the interior of the rotary kiln, cyclonesand ducts between the cyclones.
 12. The method of claim 1, furthercomprising reducing a concentration of SO₂ in the kiln by creating SO₃by using calcium sulfite (CaSO₃) as an intermediate catalyst.
 13. Anapparatus for producing cement clinker using high sulfur fuels and rawmaterials, comprising:a rotary kiln having a burner and configured tosinter the raw materials to produce sintered material; a coolerconfigured to cool the sintered material to form cement clinker; asulfur analyzer configured to measure sulfur content in the cementclinker; and a controller configured to control the concentration ofoxygen in the kiln based on the measured sulfur content.
 14. Anapparatus according to claim 13, further comprising an exhauster,wherein the controller is configured to control the concentration ofoxygen in the kiln by controlling the speed of the exhauster.
 15. Anapparatus according to claim 13, further comprising a precalciner forprecalcining the raw materials using high sulfur fuel at a precalcinerburner before the raw material is sintered in the kiln.
 16. An apparatusaccording to claim 13, further comprising:a cyclone suspension preheatersystem for preheating and decarbonating the raw material before the rawmaterial is fed into the kiln; an exhauster for pulling air trough thekiln, precalciner and cyclone suspension preheater system; a tertiaryair line connecting the cement cooler to the precalciner to permit theflow of the air from the cooler to the precalciner; a tertiary air linevalve in the tertiary air line to regulate the flow of air to theprecalciner; and an oxygen analyzer to measure the concentration ofoxygen in the precalciner; wherein the controller is configured toregulate the position of the tertiary air line valve based on oxygenconcentration in the precalciner and to regulate the speed of theexhauster motor based on the sulfur content in the cement clinker. 17.An apparatus according to claim 16, further comprising:an oxygenanalyzer at the rotary kiln's inlet to monitor oxygen concentration; apair of sulfur analyzers at the outlet to the cement clinker cooler tomeasure the content of sulfur in the cement clinker; a carbon monoxideanalyzer in the line to the exhauster to monitor carbon monoxideconcentration; and an electrostatic precipitator to remove dust from thegas passing out of the exhaust of the cement clinker cooler.